1. Field of the Invention
The present invention relates to an AD converter AD-converting vector analog signals (a set of analog signals), and to a radio receiver including the same.
2. Description of the Related Art
In a radio communication field as of 2005, many methods are adopted that modulate both amplitude and phase with a signal. For this purpose, two orthogonal signals called an in-phase signal (referred to as an I signal) and a quadrature-phase signal (referred to as a Q signal) are used in many cases.
Around 1995, for example, to form a filter on an integrated circuit, a single-ended method (a method in a circuit handling a voltage between a signal line and a ground line as a signal) was sometimes adopted for both of the I-signal and the Q signal. From 2000 onward, a differential method (a method in a circuit handling a voltage between a plus signal line and a minus signal line as a signal) has been often adopted for both of the I-signal and the Q-signal. An advantage of the single-ended method is that the single-ended method requires less number of components compared with the differential method. In a communication method in 1990s, since a transfer rate was low and large capacitors were required, some of the capacitors were mounted on a circuit board as external components to/from the integrated circuit. To reduce cost, the number of external components needs to be reduced as much as possible, and thus the single-ended method is more preferable.
In the single-ended method, however, it is necessary to supply amplifying circuits in the integrated circuit with an analog ground potential, in addition to a power supply voltage and a ground potential, as an analog reference voltage. In this case, since a signal current flows to the analog ground potential, a buffer amplifier for analog ground with a high current supply (or current absorbing) capability has to be used. If output impedance of the buffer amplifier for analog ground is high, the analog ground potential varies due to the signal current. This variation is a cause of, for example, signal leakage from the I-signal to the Q-signal, or a cause of signal leakage from an output to an input. The latter signal leakage causes a problem of the oscillation of the circuit.
From 2000 onward, in a radio communication method, a band range has become wider for higher-speed data transfer and relatively small-capacitance capacitors have come in use. It has become possible to integrate most components on a chip, which has promoted more use of the differential method. In the differential method, voltages equal in magnitude and reverse in polarity are supplied to a plus terminal and a minus terminal respectively for amplification or the like. An average value of the voltages of the plus terminal and the minus terminal virtually serves as an analog ground potential, but there is no need to provide an analog ground terminal because a current outputted from the plus terminal flows to the minus terminal. Not requiring the buffer amplifier for analog ground that is necessary in the single-ended method, the differential method requires less power consumption.
At present, the differential method is adopted in most cases. To convert an analog signal to a digital signal, a differential amplifying circuit is also often used as described in the specifications of U.S. Pat. No. 6,031,480 and U.S. Pat. No. 6,753,801.
However, as cost per area of a semiconductor chip increases in accordance with the progress of microfabrication technology, analog circuits, especially, passive elements have come to occupy a large ratio in chip area, that is, in chip cost. Therefore, to reduce cost, a reduction in area of the passive elements is an important issue.
The single-ended method is advantageous in reducing chip area and cost because the single-ended method requires a less number of components, but has a problem of increased power consumption because the single-ended method requires the buffer amplifier with a high current driving capability in order to supply the analog ground potential to circuit blocks. To solve this problem, the inventors have tried to devise an analog 3-phase signal processing circuit having an amplifier, a filter, and a frequency converter circuit.
Similarly, for an analog-digital (AD) converter, a method not requiring an analog ground potential with a high current driving capability and capable of reducing cost is necessary.
[Related Art 1] U.S. Pat. No. 6,031,480
[Related Art 2] U.S. Pat. No. 6,753,801